We are studying the arrangement, replication and function of DNA sequences in D. melanogaster chromosomes. The research on arrangement and function is based on our ability to isolate homogeneous populations of individual segments of chromosomal DNA to which we apply the techniques developed for study of viral DNAs. Isolation has been accomplished by cloning hybrid DNA molecules consisting of a single segment of D. melanogaster DNA inserted into a plasmid DNA which can replicate in E.coli. A library of different hybrid plasmids is being collected and classified with respect to topography of certain relevant sequences - e.g., repetitive and nonrepetitive, and transcribed and translated sequences. Techniques for the isolation of specified genes and base sequences are also under development. Our studies on the fine structure of replication forks, fork rate and the distribution of replication origins in the chromosomal DNA of cleavage nuclei are now being extended to include the highly repetitive satellite DNAs found in the centromeric heterochromatin.